More than 70 firefighters have worked through the night to contain a fire that threatened houses and a pine plantation near Mount Pleasant in the Adelaide hills.

Water bombers helped tackle the fire, which is thought to have been lit deliberately.

The outbreak burnt nearly 100 hectares of grass and scrub between Mount Pleasant and Tungkillo.

Parts of the Adelaide-Mannum Road and the Birdwood-Mount Pleasant Road remain closed.

Carmen Lewis from the Country Fire Service says the area is dangerous.

"The most significant risk right now is the number of trees that have fallen across the roads and the potential for more to fall," she said.

"There are a significant number of large trees in the area and internally they could still be burning, so there needs to be an assessment made of the area and once it has been deemed safe, crews can get in there and start to clear away those roads."

A fire in the Onkaparinga Hills is also being treated as suspicious.

Water bombers were used to help contain the blaze, which burnt 11 hectares late on Monday.